1. Field of the Invention
The present invention relates to valve restrictor devices that are intended to control the flow rate of gases flowing through a conduit at elevated temperatures and pressures. More particularly, the present invention is related to valve restriction devices used to regulate the flow of low molecular weight gases, such as hydrogen, that are super saturated with water vapor.
2. Description of the Prior Art
The prior art record is replete with different types of flow restrictor mechanism that can be used to control the flow rate of gas in a conduit. Many such prior art flow restrictors rely upon elastomeric materials within the structure of the restrictor to create gas impervious seals. However, in industry there are many applications where different gases are used at elevated temperatures and pressures. In such applications, flow restrictors with elastomeric seals either melt or vaporize, thereby causing the flow restrictor to fail.
The control of high temperature gas at elevated pressures is even more problematic when the gas being controlled has a low molecular weight, such as hydrogen gas. With a low molecular weight gas, even the smallest of leaks in a seal can result in significant flow rate changes. If the gas being regulated is super saturated with water vapor, flow restrictors with very small flow apertures can easily become clogged. This too creates significant flow rate changes. Many prior art flow restrictors therefore cannot be used because they are incapable of creating a seal that can withstand gases at high temperatures and pressures without failing or clogging.
One specific process that uses low molecular weight gas at high temperature and pressure is the process of purifying hydrogen gas. There are many different ways to produce hydrogen. However, in many common processes that produce hydrogen, the hydrogen gas produced is not pure. Rather, when hydrogen is produced, the resultant gas is often saturated with water vapor and contaminated with hydrocarbons and other contaminants. In many instances, however, it is desired to have ultra pure hydrogen. In the art, ultra pure hydrogen is commonly considered to be hydrogen having purity levels of at least 99.999%. In order to achieve such purity levels, hydrogen gas must be actively separated from its contaminants.
In the prior art, one of the most common ways to purify contaminated hydrogen gas is to pass the gas through a separation conduit made of a hydrogen permeable material, such as palladium or a palladium alloy. As the contaminated hydrogen gas passed through the separation conduit, atomic hydrogen would permeate through the walls of the conduit, thereby separating from the contaminants. In such prior art processes, the separation conduit is kept internally pressurized and is typically heated to several hundred degrees centigrade. Within the separation conduit, molecular hydrogen disassociates into atomic hydrogen on the surface of the separation conduit and the separation conduit absorbs the atomic hydrogen. The atomic hydrogen permeates through the separation conduit from a high pressure side of the conduit to a low pressure side of the conduit. Once at the low pressure side of the separation conduit, the atomic hydrogen recombines to form molecular hydrogen. The molecular hydrogen that passes through the walls of the separation conduit can then be collected for use. Such prior art systems are exemplified by U.S. Pat. No. 5,614,001 to Kosaka et al., entitled Hydrogen Separator, Hydrogen Separating Apparatus And Method For Manufacturing Hydrogen Separator.
In order to keep gas within the separation conduit at an elevated pressure, a restriction is placed at the venting end of the conduit. The restriction allows only a small volume of hydrogen from exiting the separator conduit in a given period of time, thereby causing the hydrogen in the separator conduit to remain at an elevated pressure.
Since the gas being restricted is mostly hydrogen, relatively small openings in the restriction create significant flow rates. In certain prior art separators, the restriction currently being used is a metal plate obstruction having 0.0008 inch pin aperture formed through the center of the obstruction. An aperture of that size is rated to create a flow rate of approximately 140 cubic centimeters per minute of helium at 400 degrees celsius and 140 p.s.i. However, when such an aperture is used in a hydrogen separator, the small aperture tends to become clogged by condensing water vapor and other contaminants that are present in the hydrogen gas. Additionally, such prior art flow restrictors are not adjustable. Consequently, if it is desired to change the flow rate, the flow restrictor must be replaced.
A need therefore exists in the art for a flow rate restrictor that can restrict the flow of gases having low molecular weights and flowing at elevated temperatures and pressures. A need also exists for a gas flow restrictor that can restrict low molecular gas containing contaminants without clogging.